(57) A frequency lock system including an oscillator (12, 13, 21) providing a switching output voltage that switches at a given frequency between fixed periods in each of first and second states and a reference source (34) providing a variable reference voltage that periodically varies from and returns to a given voltage level at a predetermined frequency less than 1/2 the given frequency. A reference lock circuit (37) provides a reference pulse of finite width in response to occurrence of the given voltage level and an oscillator lock circuit (44) provides a control pulse of finite width a certain period after switching of the output from its first to its second state. In addition, a reset circuit (43, 45) resets the oscillator (12,13,21) to initiate a new fixed period thereof at the second state in response to temporal coincidence of the reference and control pulses.